Switching and display elements in which ferroelectric liquid crystals are used as switching and display medium (FLC displays) are described, for example, in U.S. Pat. No. 367,924. They contain a layer of a ferroelectric liquid-crystalline medium (FLC) which is enclosed on both sides by electrically insulating layers, electrodes and boundary plates, normally glass plates. In addition, they contain a polarizer if they are operated in the guest-host mode and two polarizers if they are operated in the birefringence mode. The electrically insulating layers are intended to prevent electrical short circuits between the electrodes and the diffusion of ions from the glass of the boundary plates into the liquid-crystalline layer. Furthermore, at least one and preferably both of the insulating layers act as orienting layers which convert the liquid-crystalline material to a configuration in which the molecules of the liquid-crystalline layer lie with their long axes parallel to one another and in which the smectic planes are arranged perpendicularly or at an angle to the orienting layer. In this arrangement, there are two possible and equivalent orientations for the FLC molecules into which they can be brought by pulse-type application of an electric field. They always remain in the last orientation produced even if the field is switched off or the display is short-circuited. FLC displays are therefore capable of bistable switching. The switching times are in the region of .mu.s and are the shorter, the higher the spontaneous polarization of the liquid-crystalline material used is.
Compared with the liquid-crystal displays hitherto used, which are not as a rule ferroelectric, FLC displays have, in particular, the advantage that the achievable multiplex behavior, i.e. the maximum number of lines capable of selection in the time-sequential technique ("multiplex technique") is very much larger than in the known non-ferroelectric displays.
A disadvantage of FLC displays may, however, arise from the fact that a display which has been in one of the two stable states (stationary image) for a prolonged time can only be switched over with very great difficulty, i.e. with very high amplitude or very long pulse duration of the applied voltage, to the other state, that is to say it exhibits a pronounced optical hysteresis. As a result, in displays an image which has been written in for a prolonged time can be detected in outline in the subsequent image as a so-called "ghost image". This phenomenon of optical hysteresis is the more pronounced, the higher the spontaneous polarization of the FLC material. Since, on the other hand, the switching time of FLC materials is inversely proportional to the spontaneous polarization, an important advantage of FLC displays is nullified again by this hysteresis. The cause of this phenomenon has so far not been definitely clarified; there are indications that ionic impurities in the FLC are responsible for it.